Timing circuit



April 0, 1946. P. G. EDWARDS 2,399,213

TIMING CIRCUITS Filed July 18, 1944 ro oJA RD POTENTIAL lNl/ENTOR M RGEDWARDS ATTORNEY i Patented Apr. 30, 1946 I a r 2,399,213

UNl'lED STATES PATENT oi-"r cs 2,399,213

TIMING omcm'r' Paul G. Edwards, Verona, N. 1., assignor to Bell Telephone Laboratories, Incorporated, New York, N. Y., a corporation of New York Application July 1-8, 1944, Serial No. 545,511

I 5 Claims. -This'invention relates principally to timing circuits involving the use of condensers for introducing large time constants, and particularly the condenser plates to the. device to be con-- trolled. Frequently the high side of the charged condenser. is connected to a contact element of a relay, or other switching apparatus, so that when the relay or switching apparatus is subsequently operated the-condenser-is connected'to the device to be controlled and its; harge is uauua to ener size and operate the device.

. The contacts or the relay or switching appara tus are generally insulativelymounted on the frame of the relay so thatthere is a'relatively high impedance leakage path from the contacts to the.

frame and from the irame to ground. Obviously, such leakagepath is connected directly in shunt with the condenser so that the charge on the condenser tends to leak. of! toground over such path. .When the relay, or switching apparatus, eventually operates to transfer the condenser charge to the device to be operated thereby, it is apparent that the charge available on the condenser will have been reduced, it not entirely dissipated through the leakage path with the result that the potential at the condenser terminals may be insufficient to eflect the operation of the device to be controlled thereby.

It is the object of thi invention to provide means whereby the charge on a condenser may be sustained for an indefinitely long period of time;

This object is attained in accordance with a feature of the invention by applying at a pointin the leakage path, over which the charge on a condenser would otherwise be dissipated. a guard potential or substantially the same magnitude as that of the source from which the condenser is charged, so-that the .diiierence in potential between the high side of the condenser and the guard point is substantially are and therefore, the

vary automatically with variations in the potential of the source from which the condenser is charged, Thus, irrespective of variations in the potential of the charging source, the corresponding charge applied to the condenser will be sustained for an indefinitely long interval of time.

a In this aspect of the invention, the condenser is charged from a variable signaling source and the charging signal is simultaneously applied to the control electrode of a vacuum tube amplifier, so that as the charging signal is varied the output of the vacuum tube is correspondingly varied and a corresponding potential is fed back from the output of the tube to the guard interposed in the leakage path. In this manner the potential applied to the leakage path follows closely the charge potential applied to the. condenser, and for all values of potential to which the condenser :is charged the difference of potential between the "high side of the condenser and the point at which the guard potential is applied to the leakage path is-aut matically maintained at'substantially zero value and the flow of leakage current will therefore be maintained atsubstantially zero value.

These and other features of the invention will be readily understood from the following detailed description when read in connection with the-ac-' v companylng drawing, in which Fig. 1 illustrates a circuit diagram involving the fundamental principles of the invention; and

Fig. 2 is a. schematic view of the contactsof a switching device to which the guard is applied and shows how the leakage is prevented after the ilow of current over the leakage path is alscof switching contacts have been opened.

At III, in Fig. 1 is diagrammatically indicated a source of current, in the nature of a. battery, having a potentiometer [2' connected thereacross. The battery Ill and potentiometer i2 are employed for illustrative purp se. only since it is obvious that any other suitable source of variable potential maybe employed. A switching device in-the form ot a relay, is schematically shown at l3 whose armature controls the application of the potential from the source Hi to the condenser ll. This relay may be controlled in any suitable manner to efiect the. closure of its armature contacts and to thereby connect the condenser H to the charging source.

y The reference numeral in practice, may be employed asthe medium by which the charge acciunulated on condenser N may be transferred to a work circuit or other devieeto be actuated by the condenser charge. In such a case, the high side, or upper terminal g l5 identifies the irame or a relay or other type of switching device which,

of condenser H would be connected to an open contact of the switching device which when moved into engagement with its related stationary contact would function to transfer the condenser charge to the circuit or apparatus associated with the said' related contact of the switching device. Such contacts, invariably, are insulatively mounted on the frame structure of the relay or switching device, such frame being identified on the drawing bythe reference numeral I5. The insulation between the open contact connected to the high side of the condenser presents a. leakage path from the high side of the condenser to the relay frame l5 and this path is indicated by the leakage resistance is. Similarly, between the relay frame l5 and ground there is also a leakage path H. The leakage path l6, ll obviously is in shunt with the condenser H and therefore provides a path over which leakage current would travel and over which the charge on the condenser would be dissipated if precautions to guard against such leakage were not taken.

At I8 is illustrated a three electrode vacuum tube cathode follower amplifier whose control grid I8 is permanently connected to the high side of the condenser 14, so that whatever signal from the source Ill is applied to the condenser for charging purposes is simultaneously applied to the control electrode of the tube I8. The output of the tube l8, therefore, varies with the variationsin the potential applied to the condenser, in other words, with variations in the potential of the charging source.

The plate electrode of tube i8 is connected to the plate battery 2|, whereas the cathode 22 is connected to the battery 23 by way of the resistor 24. The cathode 22 and the frame ii are interconnected through the battery 25 and variable resistance 26 so that the output potential of the tube I8 is fed back to the frame or guard 15. The elements 25 and 26 supply the usual bias which is required to counteract the normal voltage between the grid Iii-and cathode 22 assumed by the tube.

With the connections described; it is apparent that a voltage is fed back to the frame or guard l5, which voltage varies with the output of the tube l8. Since the tube output varies in accordance with the potential applied to the grid l9 of the tube, which, in turn varies with variations in the charging potential applied to the condenser 14 incident to the operation of relay I3, it follows that the guard potential applied to the leakage path at the point l5 also varies with the potential of the charging source. Therefore, as the potential of the charging source changes, so also does the potential applied to 15. The voltage fed back to the guard, or frame I5 is made as nearly equal to the charging voltage as possible. Obviously, the inequality in the guard and charging voltages determines the extent to which the condenser M, will discharge through the leakage path. If this inequality is zero, then the condenser, insofar as leakage re-- sistance I6 is concernedwill remain at the voltage of the charging source, If the inequality is not zero, then the condenser will charge or discharge at a rate determined by the inequality and where the inequality is smallthe rate of change would be slow.

When relay l3 isoperated to close itsarmature contacts, a signal from the battery III is impressed on the condenser l5 and simultaneous- 1y on the grid IQ of tube It. The magnitude of the potential of this signal depends upon the setting of the potentiometer pointer and may vary from the full potential of battery [0 to any lower value. The condenser H is accordingly charged to the potential of the impressed signal. Without recourse to the. guard potential, this charge on the condenser would leak off to ground over the leakage path represented by resistances l6 and H, the rate of leakage depending upon the value of the resistances I6 and I1 and the capacity of the condenser H. To preclude the possibility of the condenser charge leaking off, the guard or relay frame I5 is maintained at a potential substantially equal to that of the high side of the condenser I4, so that the potential difference between he high side of the condenser and the guard is zero. Therefore, there will be .sipated thereover.

no current flow in the leakage resistance Ii since there is no potential difference between its terminals. Thus the charge on the condenser is sustained, and is maintained at the potential of the charging source. Since the voltage applied to the guard in the leakage path is a measure of the output of tube 18 which, in turn, varies with the signal voltage applied to the control electrode I9 and to the condenser H, the guard voltage will, for all values of signal voltage, vary correspondingly, with the overall effect that for all values of chargevoltage the difference of potential between the high side of the condenser and the guard will be substantially zero and there will be no leakage from the condenser over the leakage path. In this manner the charge on the condenser is maintained automatically irrespective of the magnitude of the potential of the charging source and for each change in the potential of the charging source the guard'potential experiences a corresponding change.

The amplifier output is obtained between the terminals 29 and may be employed for other useful purposes.

The schematic showing in Fig. 2 illustrates the guard member I5 interposed between the operating contacts of a switching device and ground. When the switching device is operated the con tact member 30 engages the stationary contact member 3| to cause the condenser II to be charged from the charging source indicated as connected to the contact spring 30. When the contact members 30 and 3| are disengaged the charge accumulated on condenser I4 would find a leakage path to ground over the insulation resistances I6 and and the charge would be dis- By maintaining the guard member l5 at a potential substantially that of the charging source, as described above, leakage of the condenser charge over the leakage path is prevented and the charge on the condenser is maintained indefinitely.

It is apparent that this invention provides a simple arrangement for obtaining the effect of a large time constant. The time constant obtainable is multiplied by l/A, where A equals l-K where K is the ratio of the original or applied voltageto the voltage fed back to the guard.

What is claimed is:

1. In combination, a condenser, means for charging said condenser to various potentials, a leakage path over which the charge on said condenser may be dissipated, a guard included in said leakage path, and means for applying potentials to said guard which vary automatically with variations in the potential ofsaid charging ing its input associated withsaid charging means and its output connected to' said guard.

2. In combination, a condenser, a variable potential source ior charging said condenser and having a connection to one platethereot, a leakage path over which the charge on said condenser may be dissipated extending irom the said one plate of said condenser, a guard included in said leakage path, and means for maintaining a zero difference of potential between the said one plate of said condenser and said guard for all values of potential of said charging source comprising a space-discharge device including an input circuit and an output circuit, means connecting the input circuit or said space-discharge device to the said one plate of said condenser and to said charging source, and means connecting the output circuit to said guard.

3. In combination, a condenser; means for charging said condenser to variable potentials, a leakage path over which the charge on said condenser may be/dissipated. a guard included in said leakage path, and means ipr maintaining a potential on said guard which varies automatically with the potential of said charging means. comprising an amplifier tube having its input circuit connected to the output of said charging means and to a point on said leakage path, and

means connecting said guard to a point in the output of said amplifier tube.

4;. In combination, a condenser, means for charging said condenser comprising a generator of signals oi variable potentials, a device included in circuit with said condenser and said generator and having a signal current leakage path over which the charge on said condenser when sub- Jected to signals irom said charging means may be dissipated, means for precluding the dissipation of the charge onsaid condenser over said leakage path comprising a guard associated with said device, and means for maintaining said guard at a potential which automatically varies with the variable potential signals of said charging means.

5. In combination, a condenser, a signal generator adapted to supply charging signals of variable potentials to said condenser, a leakage path over which the charge on said condenser may be dissipated, said leakage path being connected in shunt with said condenser, and means ior precluding the loss of signal resulting from the leakage of the condenser charge over said leakage path comprising a guard interposed in saidleakage path, and an ampliiier having an input circuit and an output circuit, said amplifier having its input circuit connected to said generator and to said condenser, and having at least a part of the output voltage or its output circuit applied to said guard to maintain said guard at substantially the instantaneous signal potential of the signals supplied to said condenser by saidsignal gen- I0 erator.

PAUL G. EDWARDS. 

